Field of the Invention
This invention relates to the measurement and characterization of the emissivity of objects, and more particularly to emissivity characterization across the MWIR and LWIR spectral bands and from approximately 50 C to 300 C and higher.
Description of the Related Art
Objects at temperatures greater than 0 Kelvin emit electromagnetic energy (light) in some relationship to Planck's Law. If an object emits energy perfectly in accordance with Planck's law, the object is said to be a “blackbody.” Emissivity is the measure of how efficiently an object emits energy with respect to Planck's law. Emissivity is a unitless quantity between 0 and 1 computed as the ratio of blackbody radiation to the object's radiation.
For example, if an object emits energy perfectly with respect to Planck's law, its emissivity would be 1. If another object were only 50% efficient at emitting energy with respect to Planck's law, its emissivity would be 0.50. The emissivity properties of a material can vary as a function of the object's temperature and the wavelength at which the energy is emitted.
Understanding this quantity is extremely important to many fields, including infrared imaging, optical design, remote sensing, etc. In the case of infrared imaging, knowing a material's emissivity allows for the calculation of the material's temperature without the use of temperature sensors or thermometers. The optical designer desires optical materials to be made of low emissivity materials that do not give off unwanted energy when hot. Materials with very high emissivity are used as standards or “blackbody simulators” to calibrate remote sensing equipment.
Characterizing the emissivity of a material with high emissivity is relatively straightforward. Characterizing transmissive, low emissivity materials like optical windows or lenses is more difficult, and even harder at high temperatures. The energy emitted from such materials is very low, hence difficult to separate from other sources of radiation such as the heat source, ambient etc.
A method for characterizing the emittance of a transmissive, low emissivity sample involves heating the sample in an oven until it reaches the desired temperature, removing the sample, and using a radiometer to measure the energy emitted by the sample. Once removed, the sample's emissions decrease as a function of temperature. To characterize the emissivity accurately, it is necessary to know the sample's thermometric temperature quite accurately at the precise time the radiometer takes a measurement. This is quite difficult in practice. Furthermore, the cooling process results in an uneven temperature across the sample; further reducing accuracy.
Another method for measuring the emissivity of a sample at high temperatures is to measure the sample radiance in-situ through an optical window in the oven. This eliminates the cooling problems associated with the previous technique, but adds additional problems. To heat the sample, the ambient temperature inside the oven is very high. The oven emits a large amount of energy that tends to overwhelm the miniscule energy emitted by the sample. The sample energy is “in the noise” of the oven emissions, and thus very difficult to extract. The window restricts the wavelengths that the emissivity testing can be conducted over. A specially chosen window must be selected to measure emittance over different wavebands. This prevents broadband or hyper spectral measurements using this technology.